The present invention concerns a method in a press section of a paper machine, in particular in a so-called closed press section provided with a smooth-surfaced press roll, for the control of detaching of the paper web from the press roll.
The present invention further concerns a device in a press section of a paper machine, the press section including a smooth-surfaced press roll, preferably a central roll, with a web being detached from the smooth surface thereof, and preferably passed as an open draw to a drying section of the paper machine.
So-called closed press sections are commonly used in a paper macine, wherein one press nip is formed or generally several press nips are formed in connection with the central roll. An example of a prior-art press section is a press section marketed by the assignee under the trademark "SYM-PRESS II", where a smooth-faced central roll having a larger diameter than diameters of other press rolls, is usually made of rock, as a rule of granite. Since granite is an unhomogeneous natural material of low tensile strength, it is quite questionable in machine construction. For example, if a granite roll is desirably heated, the deformations thereof which are dependent upon temperature, are non-linear and difficult to predict.
As press roll material, granite has relatively good properties of adhesion, transfer, and detaching of the web, which are several of the reasons for its repute. However, the detaching properties could be better, especially with regard to unbleached paper qualities.
In a manner known in the prior art, the web is detached as an open, unsupported draw from the face of the central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In the open draw, a difference in speed is used which extends the web, resulting in certain drawbacks. Moreover, the open draw forms a questionable point, susceptible to breaks in a paper machine.
Prior art technology has not provided efficient means for controlling the open draw of a web which occurs from a smooth-surfaced central roll. The unfavorable properties of granite have, for their part, make control of the open draw more difficult.
The open draw of the web has become an increasingly difficult point, with continuously increasing running speeds of a paper machine. Since different paper qualities are often manufactured by way of a single paper machine, with adhesion to the surface of a rock roll being different for different paper qualities, variations in detaching tension required for a web result.